In general, a syringe is composed of an outer tube, a gasket slidable in the outer tube, and a plunger for moving the gasket.
Conventionally, furthermore, for example, a gasket made of an elastic material such as vulcanized rubber or thermoplastic elastomer has been widely used.
Such a gasket has a large sliding resistance between the gasket and the inner peripheral surface of the outer tube, so that lubricant has been applied on the surface of the gasket or the inner peripheral surface of the outer tube to decrease the sliding resistance between the gasket and the inner peripheral surface of the outer tube.
However, such a syringe has the following problems, for example, when a drug solution or the like is contained in the outer tube.
(1) There is a possibility that in the drug solution contained in the outer tube, the lubricant applied on the surface of the gasket or the inner peripheral surface of the outer tube is mixed, which may affect the medicinal ingredients of the drug solution.
(2) There is a possibility that the medicinal ingredients of the drug solution contained in the outer tube are adsorbed (bonded) to the lubricant applied onto the surface of the gasket or the inner peripheral surface of the outer tube.
For solving the conventional problems described above, it is conceivable that a gasket is composed of a gasket body and a film covering at least the outer periphery of the gasket body in which the film is formed of a material having a friction resistance smaller than the material of the gasket body to omit the application of lubricant on the surface of the gasket.
In this kind of the gasket, however, depending on the surface qualities (conditions) of the film or the material of the film, there is a possibility that the film is peeled off of the gasket body or sealability and slidability with respect to the inner peripheral surface of the outer tube of the syringe decrease. Furthermore, depending on the thickness of the film, there is a possibility of causing a molding defect such as pinholes.
The inventors of the present invention have found out that these disadvantages could be solved by properly selecting the surface qualities of both surfaces of the resin film for covering the gasket body, and material and thickness of the resin film, and thus the gasket of the present invention has been finally completed.
Furthermore, in the gasket described above, there is a possibility that a step of preparing a film or a step of coating a gasket body with the film is complicated. In these steps, there is a possibility of causing a molding defect such as pinholes.
The inventors of the present invention have found out that these disadvantages could be solved by properly selecting the method of preparing a resin film and the method of coating the gasket body with the resin film, and thus the method of manufacturing the gasket of the present invention has been finally completed.